The present invention is of an epidural needle guide device and, more particularly, of a modification in currently known epidural injection systems which allows the anesthetist to properly locate the needle tip in the epidural space more automatically with less reliance on the needle's feel via the operator's hands.
Epidural anaesthesia is well-established as a substitute for, or an adjacent to, general anesthesia for operative surgery. Epidural block at the lumbar and lower thoracic level is suitable for most gynecological and urological work, for some general abdominal and orthopedic surgery.
To administer epidural anesthesia the anesthetist must first locate a needle in the epidural space. A number of techniques for guiding the needle tip into the epidural space are in current use. All techniques are based on the difference in resistance offered by the various tissues through which the needle passes.
Perhaps, the most widely used needle in epidural anesthesia is the Tuohy needle, which has a shaft which is usually 8 cm long. The needle wall is thin so that the lumen will admit a catheter of reasonable size. A stilette prevents coring of superficial tissues and increases rigidity so that the needle does not bend when re-directed. The point is relatively blunt and is contoured so that a catheter emerges at an angle of about 20 degrees. The needle may be equipped with a winged hub which may provide better control.
Proper placement of the needle tip is generally determined by the feel of the needle as it travels through the tissues on its way to the epidural space. The needle first pierces the skin, then travels through superficial tissues, then through the supraspinous ligament, the interspinous ligament, the ligamentum flavum, and finally into the epidural space.
To test the location of the needle, one could, for example, do the following. With the needle inserted perhaps 2 cm into the patient's back, it is useful to test its `feel` in the tissues by moving the hub of the needle up and down. Since the tip is able to move relatively freely in the superficial tissues, the needle moves like a see-saw, the fulcrum being its point of entry at the skin. When the needle is advanced further, the tip eventually engages in the ligamentum flavum and if the hub is again moved up and down, the `feel` is very different since the needle is now fixed at two points along its length and behaves like a springboard with restricted motion.
Because of their different densities, the intervertebral ligaments offer varying degrees of resistance to both the advance of a needle and to the injection of liquid. Identification of the epidural space may depend on the fact that saline, being incompressible, cannot be injected if the needle tip is in the ligament. Once the epidural space is entered, both the advance of the needle and the injection of saline become easier since the space contains only loose tissue.
The whole needle-syringe assembly is advanced slowly while steady pressure is applied to the plunger with the thumb. Entry to the epidural space is marked by the sudden ability to inject saline, 5 ml of which is usually sufficient to establish that the needle is correctly placed. The technique suffers from the disadvantage of being entirely dependent on the feel of the anesthetist. An error in judgment on the part of the anesthetist can lead to the penetration of the dura and lead to serious and undesirable medical complications for the patient.
Traditionally it has been believed that there is a sub-atmospheric pressure within the epidural space due to transmission of the intrapleural pressure through the intervertebral foramina. It has also been suggested that the pressure is due simply to `tenting` of the dura by the point of the needle. Whichever is the case, air or liquid will tend to be sucked in through the needle when its tip enters the epidural space.
Two techniques have been described to take advantage of the under-pressure. In the hanging drop method (Soresi, Peridural Anesthesia: A Preliminary Report, Medical Record (New York) 35: 165-166 (1932)), a drop of fluid is applied to the hub of the needle once its tip is embedded in the ligaments. As the tip enters the space, the drop of fluid is sucked in.
In the balloon method (Macintosh, New Inventions 2: Extradural Space Indicator, Anaesthesia 5: 98 (1950)), a small balloon is attached to the needle and is inflated through a side port. As the needle tip enters the space the balloon deflates.
Both of these techniques suffer from the disadvantage of being complicated and less than reliable.
There is thus a widely recognized need for a method of guiding the needle tip of an epidural anesthetic needle into the epidural space which does not depend heavily on the feel and judgment of the anesthetist.
It would be desirable to have a method of guiding the needle to its proper place quickly and efficiently and semiautomatically without fear of falling short of the epidural space or of overshooting it and penetrating tissues lying beyond the epidural space.